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Shoulder injuries sap power/bat speed. Justin Upton '10 is a good example. Hanley/Rendon '11.
Hanley was a mess of injuries in '11. Back and leg pain so terrible that he couldn't move at some points during the season. He aggravated the shoulder in '10 and it worsened late '11 (leading to surgery).
PEDs are always a possibility, but why stop this year? Regulations changed from "04-'05. '12 is another year that could lead to more collapse, but I see no reason for a roider to stop if they've been bypassing the (weak) system since '05.